This invention relates to separators and, in particular, to separators for use in metal halogen electrochemical cells.
In cells of the above type such as, for example, zinc bromine batteries, a separator is disposed between the positive electrode assembly and the negative electrode assembly. These assemblies are each fed with a circulating electrolyte and the separator main area in line with the ionic migration between the electrodes acts to retard migration of elemental halogen therebetween and, therefore, maintain satisfactory coulombic efficiency.
The periphery area of the separator is sandwiched between corresponding periphery areas of the electrode assemblies and provides a seal for preventing the circulating electrolyte feeding one electrode assembly from reaching the circulating electrolyte feeding the other electrode assembly. This peripheral area also seals the electrolyte in the cell so as to prevent escape to the outside environment. In conventional practice the electrode assembly peripheries are provided with ribs which, upon application of pressure to the assemblies by end plates, indent the separator periphery area to effect the needed seal. Typical separators used in the above way comprise a polymer material, such as, for example, a high molecular weight polyethylene, formed into a sheet and provided with a filler such as, for example, silica which establishes a porosity for the separator of about 50 percent.
For higher voltage electrochemical cells, using the aforementioned type separator in conjunction with the described sealing techniques results in less than an adequate seal. In particular, due to the required porosity of the separator, the sheet material exhibits little resiliency and, as a result, it becomes extremely difficult to establish a leak-free seal between the material periphery area and the sealing rib of the larger electrode assemblies. Also, it is not readily possible to realign the rib pattern created in the separator with the rib pattern of the electrode assembly after initial compression. As a result, the separator cannot be reused in other cell assemblies. Finally, the separator, once saturated with electrolyte, offers virtually no resistance to the higher voltage of the large cell. This allows shunt currents to pass through the separator between the negative and positive electrode sides causing the cell to short circuit internally.
It is therefore an object of the present invention to provide a separator which can be used in higher voltage electrochemical cells and which does not suffer from the above disadvantages.